1. Field of the Invention
The present invention relates to a device and a method for determining a time of arrival of a receive sequence in a receiver, as they may in particular be employed for digital transmission systems.
2. Description of the Related Art
So that a receiver in a digital transmission system may be synchronized with a digital signal sent out by a transmitter, the transmitter emits a digital signal known to the receiver. It is the task of the receiver to determine the exact time of arrival (TOA) of the transmitted signal.
In order to determine the time of arrival, the cross-correlation between the digital receive signal and the known digital transmit signal is calculated.
FIG. 5 shows a correlation sequence 414 plotted over time calculated from a continuous correlation of a received signal with a transmit signal stored in the receiver. The correlation peaks of the correlation sequence 414 shown in FIG. 4 indicate a correspondence of the stored transmit sequence with the receive sequence. In order to determine the time of arrival of the receive sequence from the correlation sequence 414, the magnitude maxima of the correlation sequence 414 are detected and the time of arrival of the receive signal is determined from the position of the correlation magnitude maximum. In FIG. 5, two main maxima 422, 422′ are shown, for which a trigger signal indicating the time of arrival of the receive sequence is set equal to 1. A further main maximum between the maxima 422, 422′ is not considered for the calculation of the time of arrival, as this maximum results from a multipath propagation.
If a correlation magnitude maximum in a predetermined range of correlation magnitude values or in a predetermined range of indices 1 of a correlation signal, respectively, is to be searched for, then the correlation magnitude maximum may be determined by determining the maximum of all correlation magnitudes existent in the predetermined range. If a correlation magnitude maximum is to be searched for with regard to a continuous signal, i.e. a stream of samples, then it has to be continuously assessed whether the correlation magnitude just calculated represents a valid local maximum or not. For determining such a local maximum a fixed threshold value may be used together with the rule, that a correlation magnitude maximum is present when a correlation magnitude to be assessed exceeds the threshold value.
A determination of the time of arrival depending on the main maximum value of a correlation sequence is inflexible and inaccurate, as there is no possibility of an adaptation to a changing transmission channel. Further, the main maximum of a correlation sequence is sensitive with regard to multipath propagation. This means, that the subsequent paths strongly influence the maximum, shift its position and thus corrupt the time of arrival. Further, the main maximum is prone to noise. Thus, the time of arrival of the receive sequence may be determined only very inaccurately from the correlation magnitude maximum, or an erroneous determination of the time of arrival may result, respectively, if a correlation magnitude maximum was erroneously detected. As a consequence of an erroneous determination of the time of arrival, a synchronization between transmitter and receiver may fail or an erroneous data transmission may result, respectively, if the synchronization may only be performed inaccurately, as the exact time of arrival of the receive sequence cannot be determined.